Murneek — 53 — Research in Photoperiodism 



10-20 f.c, though Hght of high intensity (natural daylight) was given for 

 1 hour of the 8 hour period. But with 2 or more hours of intense light 

 during the photoperiod, increasingly more initiation of reproductive struc- 

 tures was obtained (Parker and Borthwick, 1941). Withrow and 

 Benedict (1936) could see little difference in response of several green- 

 house annuals when the day length was extended with artificial light at in- 

 tensities from 10 to 100 f.c. Definite photoperiodic effects were secured 

 with 0.3 f.c. and the China aster reacted even to light of 0.1 f.c. Rasumov 

 (1935) observed that oats and millet, obtained from various localities, re- 

 sponded differently during their photoperiods to light intensities.* 



Studies of the effects of light of various wave lengths on photoperiodic 

 induction so far seem to have advanced very little our knowledge of the 

 mechanism of inception of the photoperiodic reaction, though this undoubt- 

 edly is one of the most crucial phases of the phenomenon. Rasumov (1933) 

 and Katunskij (1937) found that red light had a similar effect to white 

 light in flower initiation, but green, blue and violet radiations were like 

 darkness in this respect. This is but a confirmation of Klebs' observation 

 that red light is most effective in promotion of sexual reproduction. It was 

 confirmed again by Withrow and associates in 1936 and 1940. Schap- 

 PELLE (1936), however, claimed that red and blue radiations were more or 

 less equally effective for spinach, radish, cosmos, lettuce and China aster. 



In more detailed studies of the influence of wave lengths of artificial 

 light used to lengthen the daylight period (Withrow and Benedict, 1936) 

 obtained from orange and red light (650 + ni/x) the greatest photoperiodic 

 effect in pansy, stock and aster plants and little response from other wave 

 lengths. They conclude that the photoperiodic perception mechanism 

 probably has a maximum intensity at 650-720 m/x of radiation. For Baeria 

 chrysostoma all wave lengths, excepting green, were found effective in in- 

 duction of flowering. Light of different colors was obtained by passing 

 radiation from 1000-watt incandescent lamps through appropriate glass 

 filters (SivoRi and Went, 1944). 



Mechanism of Photoperiodism: — This would seem to be a formid- 

 able heading, for there is little more than circumstantial evidence, frag- 

 mentary at that, on the possible steps involved in the reaction of plants to 

 the photoperiod. Perhaps we can allow it to stand, since it, in a general 

 way, covers various and some very important phases of investigational 

 work on the subject. 



The leaves of a plant are the organs through which the photoperiodic 

 stimulus is received. Light, as regards duration and to some extent in- 

 tensity, is the activating agent. The first reaction, unknown at present, 

 probably is of a photochemical nature. There is ample evidence extant 

 that, as a result of the photoperiod, a substance of a catalytic character, of 

 the nature of a "hormone," is formed which is responsible directly or in- 

 directly, for the induction of floral primordia. This concept is based on a 

 large body of experimental records, first presented by Psarev, Moshkov 



♦According to N. J. Scully and W. E. Domingo (Bot. Gaz. 108: 556-570, 1947) both duration 

 of photoperiod and light intensity influence the formation of floral primordia in certain varieties of 

 the castor bean, a long-day plant. They differ, therefore, from Xanthium or Biloxi soybeans, which 

 are not very sensitive to differences in total radiant energy but highly sensitive to differences in day 

 length. Results with hybrids were inconclusive. 



